Control of heat transfer salts



,peratures in chemical some of the lines and in Patented May 15, 1945 UNITED STATES PATENT OFFICE v CONTROL SFER SALTS John R. Bates, Swarthmore, Houdry Process Corporation,

Pa.. assignor to Wilmington, DeL,

a corporation of Delaware No Drawing. Application September 27, 1940, Serial No. 358,705

1 Claim.

This invention relates to methods of maintaining the desirable properties of heat transfer salts, with special reference to alkali metal salts f the oxyacids of nitrogen.

Heat transfer salts with as low a melting point as possible are desirable for controlling temand other equipment. Thus, where the temperature is maintained between say 700 and 900 F. in the zone controlled by the salt, it is desirable for the salt to have a substantially lower melting point, whereby it may be melted by the steam available, which ordinarily runs 400 F. or lower. Thus when a plant is shut down and cooled it is desirable to be able to melt the salt in the lines with such steam in tracers. My associates and I have found that mixed nitrates and nitrites of one or more of the alkali metals are particularly suited to such uses, and upon this basis the first large plants employing such temperature controlling media have been installed. As a result of these installations, we have found that in use the properties of the salts change, resulting in an increased melting point, in the formation of deposits which in some instances partially plug all cases effect a reduction of the heat eflioiency of the systems and in the deposition of solid phase from the salt compositions. Accordingly, it is an object of this invention to provide economical processes for the treatment of such heat transfer salts to maintain a desired melting point thereof and to prevent the deposition of scale or other solid phase from the molten salt.

It has now been found that in the maintenance of heat transfer salts of the type described it is desirable to maintain the concentration of alkali in the salt low, in general below about 5%, and preferably between 0.2% and about 2%. This has been found to be of substantial importance in maintaining a low melting point and in preventing scale and other depositions, despite the fact that it has been found that limited amounts of alkali reduce the melting point of the compositions.

In accordance with this invention, a molten salt composition comprising principally alkali metal nitrate and. nitrite is withdrawn from a zone in which it is employed in moltenstate and is treated to effect a reduction or lowering of the alkali content of the salt and the salt 'so treated is returned to the zone.

The salt composition is employed in the molten is desired to control, aheat exchanger for ad- I justing the temperature of the composition, a pump for circulating the salt in the system and a surge tank for allowing some change in the total volume of salt in the system. Suitable piping is provided for conveying the salt from each unit to the next in the system. It should be understood that this system is merely representative of one form of system in which the molten salt may be employed, and that the present invention, which relates to treatment of such salts, is not limited to any particular type of system in which the molten salt is employed.

Salt is withdrawn from the zone of use and is treated in a reducing zone in which the alkali content of the salt is reduced or lowered and the salt so treated is returned to the zone of use. The salt is withdrawn from the zone of use, treated, and returned at such a rate that the alkali in the zone is maintained below 5% and preferably between about 0.2% and about 2%. In some instances the reduction may be carried to the complete elimination of the alkali in the reducer. By the term alkali throughout this description it is intended to include both free alkali (alkali metal oxide or hydroxides) and alkali metal carbonates.

Variousmethods may be employed, within the scope of this invention, for the reduction of the alkali. Thus, the salt may be treated to neutralize the alkali therein, as with an acid oxide. The neutralization may result in the formation of alkali metal salt of an oxyacid of nitrogen from the alkali, whereby the decomposed salt is reverted to useable salt or other salts may be formed, preferably salts which are insoluble in the salt composition. Such a neutralization is preferably conducted with the salt composition in molten state, though it should be understood that within the broad scope of this invention the salt is not necessarily molten during treatment.

To effect neutralization the salt may be reacted preferably in molten state with an oxide of nitrogen, by which term it is intended to include their acids, such as nitric oxide (NO), nitrogen dioxide (N02), or nitric acid (HNOa) The treatment of the salt composition is preferably effected while maintaining the salt in molten state, though it should be understood that in this description and in the subjoined claim by the term "molten it is intended to indicate liquid compositionsineluding those which contain suspended solids.

These neutralizing materials may be used sepa- When nitric oxide (NO) is employed the extent of treatment may be limited to eifect substantially only the neutralization of alkali. By neutralization of alkali metal nitrate-nitrite compositions with nitric oxide, control of the nitratenitrite ratio is obtained, since at least apparently, the nitrate enters into the neutralization reaction, so that during neutralization nitrite is formed both from the alkali and from nitrate.

If desired, the extent of the formation of nitrite from nitrate may be increased by more extended contact of the nitric oxide with the salt, after the alkali is reduced to a low percentage, on the order of below about 1%. Such a treatment is very desirable in that not only is the alkali content controlled but also in the same treater an unlimited reduction of 'the nitrate to nitrite ratio of the composition may be effected.

In some installations the salt composition employed is not substantially changed in ratio of nitrate to nitrite during use, and may contain substantially equimolar proportions of nitrate and nitrite ions. In such cases the use of nitrogen dioxide is of substantial advantage in neutralizing the alkali, since when the dioxide is used equimolar proportions of nitrite and nitrate are formed from the alkali, whereby the ratio thereof may be maintained substantially constant.

Nitric acid may be employed preferably by heating the acid substantially to the temperature of the molten salt and then introducing the vaporous product of heating the acid into the salt composition. The use of nitric acid is very convenient as it is readily obtainable on the market and may 7 be used to form a higher proportion of nitrate from the alkali than is obtained with the dioxide.

While nitric oxide, nitrogen dioxide, andnitric acid have been described specifically, other neutralizing materials may be employed for the formation of alkali metal salts of oxyacids of nitrogen, for instance N20: and N205.

In order better to effect control of the nitratenitrite ratio of the salt composition mixtures of compounds of nitrogen may be employed. Thus, nitric oxide and nitrogen dioxide may be employed either in mixture or serially to obtain a result intermediate between the two with respect to control of the ratio. Likewise, nitric oxide and nitric acid or nitrogen dioxide and nitric acid may be employed to effect intermediate results. In any of these cases the proportions of the reactants may be controlled relatively to effect production of the desired amount'of nitrate and nitrite. In accordance with a preferred form of this invention the reactants are employed in such amounts as to maintain the per cent of nitrate ion in the composition between about 10% and about 50%.

The alkali content of the salt composition may also be reduced by reacting it with other acid oxides, particularly those which are substanti lly non-volatile at the temperature of treatment and which, by metathesis with the alkali, form materials which are substantially insoluble in the salt composition.

Alternatively, the solid acid oxide may be employed as a packing material in a tower into which both the salt and any of the above described oxides of nitrogen are introduced, the salt and nitrogen oxides flowing either concurrently or countercurrently in the tower. Counter-current flow in the presence of such a packing is of particular advantage from the standpoint of conversation of fixed nitrogen.

In a system in which 1,500,000 pounds of a molten salt composition, formed by melting together 810,000 pounds of potassium nitrate and 690,000 pounds of sodium nitrite, is employed for controlling the temperature of a catalyst case at between 800 F. and 850 F. alkali was formed during the course of a, year in the amount of about 0.8%. By previous experience it was known that scale would eventually form in the system. After a year of operation, salt was withdrawn at a rate of 40,000 pounds per week to a tower, and neutralized to 0.2% alkali by nitric acid of 91.4% concentration which was introduced into the bottom of the tower at a rate of 453 pounds per week,

roughly a 5% excess. The salt composition so treated was returned to the system, whereby the alkali content was held at about 0.8%. Subsequently no scale was formed and no salting out occurred at a temperature above the melting point of the composition.

In another system similar to that above described the neutralization to 0.2% alkali was eflected by introducing a mixture of equal volumes of nitric oxide and nitrogen dioxide into a treating tower at a rate of 2,350 cu. ft. (measured at standard conditions), which also was about 5% excess. The alkali neutralized was substantially all converted to nitrite.

Salt was withdrawn from a system in which it was employed at about 800 to 900 'F. for heat 40 transfer, which salt had a ratio of potassium to aluminum silicate.

sodium of 1.36, contained 0.69% alkali (calculated as NaOI-I), contained 32.4% nitrite ion and contained as the remainder nitrate ion. This salt was contacted in molten state with nitric oxide at such a rate that the akali was reduced to 0.31%. Concurrently the per cent nitrite ion increased to.33.4%

A heat transfer salt which had a ratio of potassium to sodium of 1.36 and which contained principally nitrate and nitrite as the anions but which also contained 4% alkali (calculated as 'NaOH) was contacted in molten condition for /2 hour with silica gel. The alkali content had then been reduced to 2%. Similar results were effected using crushed glass and also when using an The best results are obtained when using materials having a large and active surface such as gels and the like.

This application is in part a continuation of my copending application Serial No. 319,446, 'filed February 1'7, 1940, which has been abandoned and refiied as application Ser. No. 559,226, filed October 18, 1944.

I claim as my invention:

In a process of treating a salt mixture consisting principally of alkali metal nitrate and nitrite which has been maintained in circulation in a molten state in an indirect heat transfer system and in which alkalinity and melting point have increased as a result of such use, the steps comprising contacting said molten mixture with an oxide of nitrogen until the alkalinity and melting point have been decreased and returning the molten mixture for indirect heat transfer.

' JOHN R. BATES. 

